Gas separation is an emerging technology with a rapidly developing market comprising applications like air separation for oxygen or nitrogen enrichment as well as acid gas removal and hydrocarbon recovery from natural gas streams. The economics of a membrane-based separation system depend on the gas permeability (thickness- and pressure-normalized flux) and selectivity (preferential permeation of one gas over another) of the material used. Unfortunately, there is a conventional trade-off between these two main parameters such that an increase in permeability is concurrent with a decrease in selectivity, and vice versa. This results in what is commonly referred to as an “upper-bound” to performance which is defined by polymeric materials with the highest known combinations of permeability and selectivity. It is revised to accommodate discoveries of better performing polymers and is therefore taken as a gauge of the state-of-the-art.
Strong research efforts in academia and industry are currently invested towards increasing the permeability of polymeric membranes without compromising selectivity by introducing microporosity, considered by the International Union of Pure and Applied Chemistry (IUPAC) to encompass pores less than 20 Å. A conventional technique to introduce microporosity into polymers is thermal treatment. However, this adds complexity to the membrane formation process and often results in insoluble, brittle films. Recently, a British group designed polymers with repeat units comprising a site of contortion in a rigid, wholly-fused ring backbone where there are no single-bonds about which free rotation can occur. This results in inefficient packing of chains in the solid state, trapping free volume and thus generating microporosity inherent to the polymer. These so-called polymers of intrinsic microporosity (PIMs) typically demonstrate high internal surface area, high thermal stability and, very importantly, high solubility in common organic solvents (key to forming membranes for gas separation applications) and amenability to functionalization (permitting tuning of performance for chemically interacting penetrant molecules). To date, ladder-type PIMs are conventionally prepared with a tetrahedral, spiro-carbon center by polycondensation between a bifunctional hydroxylated aromatic monomer (AA) and an activated fluorinated or chlorinated monomer (BB) to form dibenzodioxane-linking groups. Regarding gas separation performance, PIMs have found great success in producing significantly higher gas permeabilities than commercial polymeric membrane materials, but at the expense of selectivity. Therefore, commercial use has been severely hindered for important gas separation applications, such as O2/N2, CO2/CH4, H2/CH4 and others.